1. Field of the Invention
This invention relates generally to devices which trap and/or pump waste gasses from industrial processes, and more particularly to cryogenic traps or pumps (cryotraps or cryopumps).
2. Description of the Prior Art
Cryogenics deals with the production of low temperatures and the utilization of low temperature phenomenon. The cryogenic temperatures are generally considered to range from 123.degree. K. to 0.degree. K.
Gases used in cryogenic engineering are cooled to their boiling (or liquefying) points by three basic methods, namely liquid expansion, Joule-Thomson expansion, and expansion in an engine (refrigeration). After production, cryogenic liquids generally are stored in specially designed tanks using superinsulation or in Dewar vessels (double walled flasks having an evacuated space between them). Liquid air, oxygen, nitrogen, and even hydrogen can be kept for several hours in such vessels without further thermal protection. Liquid helium, however, has such a low heat of vaporization that it can be kept for any length of time only if the Dewar vessel is in turn surrounded by a similar, larger flask containing liquid nitrogen or liquid air.
For several industrial and research purposes, cryogenic pumps (cryopumps) are used to attain hard vacuums beyond the reach of mechanical pumps. Gasses will condense on a surface if the temperature is low enough, much as water vapor will condense a cold windowpane.
In one system an absorbent (such as silica gel) is bonded to the surface of a cryopanel. The pumping speeds of cryogenically cooled absorbents at very low pressure are sensitive to the amount absorbed, but independent of the depth of the absorbing material. The capacity of the material to absorb increases rapidly with decreasing temperatures. By cooling the absorbent to 77.degree. K., all gasses except hydrogen, helium, and neon can be effectively trapped.
Cryotraps, which are closely related to cryopumps, are often used to trap gasses formed as a by-product of industrial processes. A typical cryotrap of the prior art includes a liquid nitrogen refrigeration system which chills a condensation surface to cryogenic temperatures. The waste industrial gasses condense on the condensation surface, and are periodically removed therefrom in a flushing process.
A problem with the old liquid nitrogen cryotrap technology is that it consumes large quanities of liquid nitrogen, requiring frequent deliveries of that substance. Furthermore, the plumbing and facilities required to house the nitrogen are bulky and expensive.
The relatively new technology of helium cryopump refrigeration solves some of the problems of the old nitrogen cryotrap systems, but presents a few new ones of its own. A helium cryo refrigeration system includes a cold head through which high pressure gaseous helium is circulated. Typically, a compressor provides gaseous helium to the cold head at approximately 250 PSI, and recycles the effluent.
The helium cryo refrigeration system is advantageous over the old nitrogen systems in that the helium is constantly regenerated, eliminating the need for large storage vessels and frequent deliveries. Disadvantages of the helium systems include that they require many hours to reach cryogenic temperatures, and thus are not well suited for use in cryotraps which have to be periodically shut down for regeneration. Also, the helium system is not compatible with highly reactive gasses such as chlorine due to the characteristics of the materials used in the construction of the cold head and the associated condensation surfaces.
A solution to these problems would be to provide a thermal switch which insulates and protects the helium cold head from the condensation surface. One such thermal switch is disclosed in U.S. Pat. No. 3,525,229 of Denhoy which includes an inner vessel filled with liquid helium, and an outer vessel which may be selectively filled with a liquefied gas or evacuated with a vacuum pump. When the outer vessel is filled with the gas, the heat is conducted from the condensation surface via the liquefied gas to the liquid helium. When the outer vessel is evacuated the condensation surface is effectively insulated from the liquid helium.
In U.S. Pat. No. 4,432,208 of Onuki et al., a cold trap for liquid sodium is disclosed which has a double walled structure providing a volume 16 which may be filled with a heat insulating gas. U.S. Pat. No. 4,354,356 of Milner, teaches a temperature cycled cold trap provided with temperature sensors in a feed back mechanism. While the above identified patents teach useful cryogenic subassemblies, the prior art does not disclose a complete helium cryotrap system.